Weather Watch: Summer right place to start rainfall year

The 2008 calendar year was the perfect example of why the National Weather Service considers California's rainfall year the period from July 1 to June 30.

Looking at total rainfall from Jan. 1 through Dec. 31, the year looks fairly normal. The 11.11 inches that fell is just a tad above the 30-year average of 10.77 inches.

But 2008 was far from normal. It was a tale of two half-winters.

The year started off wet, went bone dry for nine months, then got very wet again at the end.

The problem with using the calendar year as the yardstick is that the calendar covers parts of two rainy seasons. Putting part of a season in one rain year and the other part in another would leave you without a true sense of the water year, said Jim Purpura, meteorologist in charge of the weather service's Rancho Bernardo office.

“We see these as separate water seasons with different characteristics,” Purpura said. “It makes sense to not split the rainfall season in the middle.”

Back East, the rainfall year corresponds with the calendar year. But the precipitation patterns there are more uniform. Much of California typically gets half or more of its annual precipitation in the winter alone, then goes many months with very little.

Rainfall in most places in the East is more evenly distributed. Drier periods there tend to be in the autumn and winter, but the contrast between the dry and wet periods is not nearly as great as it is in California.

The 2007-08 rainfall season, the period from July 1, 2007 to June 30, 2008, was wetter than normal by the end of January. February brought a little bit of rain, but still less than average. Then the tap virtually shut off. The season ended with 7.25 inches of rain in San Diego – 4.52 inches below normal.

This season has had a similar look so far, although the rain started earlier. In late December, San Diego had twice as much rain as normal for the season. But the spigot has shut off this month. If the trend continues, the city will fall back below normal again sometime in February.

The wettest day of the year was Dec. 17, when 1.6 inches was recorded at Lindbergh Field. More than an inch of rain also fell on Nov. 26 (1.05) and Thanksgiving Day, Nov. 27 (1.26). Rain also fell on Christmas Day, election day (Nov. 4) and Valentine's Day.

In terms of temperature, the year was slightly below normal. Only October and November were warmer than normal. The average temperature for the year was 63.7. The 30-year average is 64.4.

But the monthly temperature numbers are deceiving. Overall, despite two heat waves during the month, April was cooler than normal. The temperature rose to 94 April 27, which tied for the hottest day of the year with Oct. 8. May and June also had heat spells yet still finished below normal.

“When you look at the year, you see wild temperature swings,” National Weather Service forecaster Steve Vanderburg said. “Until the summer, which was very mild. Then in the fall, we returned to the wild swings.”

Strongs troughs, with cool air masses dropping down from the north, alternated with unusually powerful ridges of high pressure that brought hot weather. This kind of “high amplitude” pattern, Vanderburg said, has persisted into January. A strong ridge is responsible for the dry first 12 days of the month and the near-record heat the region has experienced the last couple of days.

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